Aqueous inks have become the major ink for an ink-jet recording use other than an industrial use because the danger of fire breaking and toxicity such as mutagenicity, which are major problems of oil-based inks, can be reduced.
However, a pigment-containing ink, which is used as an ink-jet recording aqueous ink, has the significant problem of nozzle clogging due to the aggregation/sedimentation of pigments although they are advantageous in that good light fastness is prepared. Sedimentation of pigments caused from aggregation results in the serious problems such as reduction in yield and destabilization of quality even in a production process of an aqueous pigment liquid dispersion that is an intermediate of an ink.
In order to improve this, various methods have been investigated, which use polymer-based dispersants to disperse pigments in an aqueous medium. However, it is generally difficult to stably disperse pigments over a long period only with polymer-based dispersants. In particular, there are only a few yellow pigments that are excellent in light fastness, and it is difficult to produce, by using limited types of a pigment, an aqueous pigment liquid dispersion and an ink-jet recording aqueous ink which are excellent in both light fastness and dispersion stability.
In view of such situation, the present applicant previously proposed an aqueous pigment liquid dispersion that contains Pigment Yellow 155, i.e. a bisacetoacetarylide pigment, and uses a styrene-acrylic acid-based copolymer as a dispersant (Patent Reference 1).
However, the aqueous pigment liquid dispersion obtained in Patent Reference 1 has the problem that the sedimentation of pigments cannot be sufficiently suppressed when it is used as an ink-jet ink that requires good discharging properties at a low viscosity although it has excellent color developing and light fastness that are peculiar to Pigment Yellow 155. Furthermore, in order to accept the demand for photograph image quality of high luster in recent years, higher luster needs to be realized, and these improvements have been strongly desired.
Meanwhile, the production process is disclosed, in which dispersion is performed under the presence of a diol compound when the pigment is dispersed in a liquid medium (Patent Reference 2). When this production process is used, it is said that a large dot diameter can be formed on a recording medium with a small discharging quantity. However, in the production process disclosed in Patent Reference 2 which includes no kneading step, since coating of pigments with resins is not sufficiently performed, the effects of anti-sedimentation and luster improvement were not obtained, and the problems such as the increase in viscosity due to free resins were likely to occur on the contrary.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. 2005-060419
[Patent Reference 2]
Japanese Unexamined Patent Application, First Publication No. 2004-002698 (Example B2)